Steven Matz may end up starring for the Mets for many years to come, but during his remarkable major league debut Sunday at Citi Field, the 24-year-old had to share the spotlight with someone else — his grandfather.

While the Long Island native made history in the 7-2 win over the Reds, going 3-for-3 with four RBIs while allowing two runs in 7 2/3 innings to earn the win on the mound, Matz’s grandfather, Bert Moller, may have gained even more fans.

When Matz crushed a second-inning, two-run double over center fielder Billy Hamilton, Citi Field — and, most memorably, Moller — erupted. Sitting in a suite with his family, Moller jumped from his seat and started fist-pumping. Then, when Matz hit a two-run single in the sixth inning, his grandfather displayed the disbelief in everyone’s minds, screaming and throwing his hands in the air before bouncing up and hitting himself on the head.

And with that, the Internet made a new best friend, with Moller’s irrepressible reactions captured for eternity via GIF.

“They said I went viral, and I said, ‘What the heck is that?’” Moller told The Post Monday by phone. “It was crazy. When he hit that double, it was like: ‘Oh my gosh.’ You couldn’t control your emotions. I was so happy.

“When he was a day old, he was in my arms. … He’s still a kid to me. It’s crazy.”

Bert Moller, at his home in Commack, LI, admires Monday’s Post back page.VictorAlcorn.com

Moller, 82, has been a Mets fan since the team was born. Growing up in Bay Ridge — attending PS 185, Fort Hamilton High School and Brooklyn College before joining the Air Force and spending 36 years working in the flight test department at Grumman — Moller was a diehard Brooklyn Dodgers fan, heartbroken when the team left for Los Angeles.

“We couldn’t change our loyalty to the Yankees. There was no way,” said Moller, who currently resides in Commack, LI. “Then the Mets came along, and we’ve been with them through all the years, the good years and the bad years.”

Moller’s pride in his grandson was palpable. Praising Matz’s work ethic, Moller described him as unbelievably caring, noting Matz often calls him and always offers to help with anything when he’s home.

Moller was headed Monday to Stony Brook to see the most famous of his 10 grandchildren, still wrapping his mind around the idea Matz could be one of the biggest reasons the Mets’ recent string of bad years may finally turn.

“Isn’t that something?” Moller said. “I had an expectation that he was going to do well. He told me he just had to get over those first-inning jitters. The later the game gets, the better he gets. Once we got into the second, I knew that was it. He was on his way.”